Nitrogen and sulfur are essential to proper nutrition of living plants. These chemical elements are commonly classified by agronomists as primary and secondary nutrients, respectively. Both of these nutrients are required for most important farm crops.
Conventional ammonium sulfate production is predominantly from the scrubbing, for emissions control, of stacks containing SO2. This material is then either concentrated for use as a solution fertilizer or granulated using evaporators and a granulator to solidify the concentrated slurry produced.
In 1983, the estimated annual production of by-product crystalline ammonium sulfate in the United States was about 2 million short tons. By-product ammonium sulfate is primarily produced from the production of caprolactam, from the production of methacrylates, and in the coking of coal. These by-products comprising ammonium sulfate are commonly recovered as relatively dilute solutions which must subsequently be concentrated, for example in a crystallizer, to cause precipitation of the ammonium sulfate. The resulting fine crystalline product is separated from the mother liquor, and utilized primarily in the fertilizer industry. The relatively few large ammonium sulfate crystals are separated from the plethora of small ammonium sulfate crystals, for example by screening, and may be suitable for use as direct application fertilizer or for blending with other fertilizer materials provided that there is homogeneity with respect to the particle sizes of the various materials making up the fertilizer.
The physical state of fine- or standard-size crystalline by-product ammonium sulfate is suitable for the manufacture of chemically granulated fertilizer, which is commonly referred to as ‘compound,’ ‘complex,’ or ‘co-granulated’ fertilizer. Generally, in the manufacture of such complex fertilizers, all ingredients are homogeneously combined prior to formation of particulate granules therefrom, such that each granule of the resultant fertilizer mixture contains all ingredients in a predetermined proportion, and in mechanically and/or chemically bonded units. In this manner, application of the fertilizer to the soil desirably results in substantially uniform application of the fertilizer ingredients.
Most of the crystalline by-product ammonium sulfate as recovered from such crystallization operations is not in a suitable physical state, particularly with respect to particle size, for use in ‘bulk blends’ or ‘dry mixed’ fertilizers. Fertilizers of this type consist of simple, dry, mechanical mixtures of discrete, nonbonded granules of two or more chemical compositions. The granules of different composition are blended in proportions calculated to yield a mixture of the desired overall fertilizer nutrient composition. This method of fertilizer preparation, being simple and adaptable to small production operations at or near the locations of fertilizer usage, remains popular in the United States. However, to ensure the homogeneity within a given small volume of such dry-blended fertilizers during mixing, handling, and field application, the various ingredients must have closely matched particle size distributions. Preparing blends from ingredients of unmatched particle size undesirably results in segregation of the various components during usage. Homogeneity is unlikely when segregation occurs, and undesirable agronomic effects are likely to result. In view of the need for the individual ingredients of bulk blends to be matched in particle size distribution, crystalline by-product ammonium sulfate, with its small particle size, cannot typically be incorporated properly therein.
Conventional application equipment for broadcast or row placement application of direct application solid fertilizer is designed for handling free-flowing granular type materials, such as granular or pelleted ammonium nitrate or urea. Such equipment will generally not properly handle such fine crystalline ammonium sulfate. When attempts are made to apply nongranular materials, such as crystalline by-product ammonium sulfate, with use of conventional application equipment, achieving uniform ground coverage is difficult due to the poor flow characteristics and bridging tendency of fine particles in the applicator.
Accordingly, there remains a need in the art for a system and method of producing ammonium sulfate granules having a desired average granule size, hardness, pill characteristics, and/or micronutrient composition.